Static miscible vapor environment controlled honeycombed morphology in polystyrene–b–poly(methyl methacrylate) films
Graphical abstract
Introduction
The micro– and nano–scale honeycombed porous films have unique advantages, such as uniform pore size, large specific surface area, high degree of roughness, and et al., and are widely used in the field of biomaterials [1], separation films [2], optics [3,4], and new energy [[5], [6], [7], [8]]. Hence, it remains a continuous challenge to prepare ordered porous film through adjusting the pore size and the morphology of film. In recent years, many approaches have been developed to prepare ordered porous films, such as lithography, colloidal crystal technology, and self-assembled template [[9], [10], [11], [12], [13], [14]]. However, these methods have been verified as unsuitable due to their low efficiency or high cost. On the other hand, breath figure (BF) technology provides a simple, cheap and efficient method for the preparation of micro- and nano-porous structures due to more lenient for a complex and demanding template removal process.
BF process generally involves of two steps. First, rapid evaporation of solvent from a polymer solution under a humid environment causes the temperature of the solution surface to decrease, Simultaneously, the wet vapor condenses into small water droplets on the surface of the solution. As time goes by, the condensed water droplets grow further apart and spread out uniformly on the surface of the polymer solution to self-organize into a hexagonal array under the effect of the surface convection and capillary attractive force [[15], [16], [17]]. Thus, an ordered porous film is formed after drying. During this process, the polymer dissolved in the solution is absorbed near the interface between the droplets and the polymer solution, which stabilizes the droplets and prevents their coagulation. With the volatilization of the solvent from the polymer solution and solidification of the polymers, water droplets are used as templates to form honeycombed porous films. Since Schatz etc [18] proposed the formation and growth mechanism of water droplets in polydimethylsiloxanes solution surface in 2001, the research in this field has taken a big step forward. Afterwards, many structures of polymers have been used to prepare ordered porous films via BF technolofy [[19], [20], [21], [22], [23], [24]]. Meanwhile, different substrates have also been used for film preparation. Subsequently, Connal etc [25] reported the first example of the preparation of porous honeycombed films that could contour to the surface of nonplanar TEM grids; this provided a wider field for the use of the BF technology.
Although many conditions of BF technology have been explored, the exact mechanism of this process is still unclear. Many factors can influence the morphology of the polymers during BF process. A slight change in the casting conditions will lead to a change to the morphology of the porous films. Besides the polymer and substrate, the influence of the vapor environment on the porous film morphology was also enormous. Many reports showed that the BF process was carried out under an aqueous vapor environment [26,27]; Ding [28] reported a novel research that the BF process could even be carried out under a non-aqueous vapor environment, which is neglected by many researchers. Lately, the influence of vapor environment on porous films morphology is becoming more and more concerned by researchers [29]. Under a wider application of porous films, a single vapor environment has been unable to meet the needs for controllable morphology [30,31]. Therefore, it is urgent to understand the effect of more complex vapor environment on the porous films morphology.
Here, we report a static BF technology to controll the morphology of block copolymer polystyrene–block–poly(methyl methacrylate) (PS–b–PMMA) porous films by using miscible vapor consisting of ethanol/water or acetone/water as atmospheric environment. Ethanol and acetone have low evaporation enthalpy. It is easy to control the condensation and the droplet growth speed through the component and the ratio of ethanol or acetone to water. We will examine the influence of miscible vapor environment on the morphology of PS–b–PMMA, homo–PS, and homo–PMMA films. The porous films formed under mixed vapor environment of water and ethanol have higher porosity. We will also pay attention to the importance of hydrophobic PS block in the formation of pores. The pore size depends strongly on the total molecular weight and the proportion of hydrophobic component PS in PS–b–PMMA. A large total molecular weight and a low PS proportion take a large pore size.
Section snippets
Materials
Polystyrene–block–poly(methyl methacrylate) (PS–b–PMMA) and homopolymers (homo-PS and homo-PMMA) were purchased from passkey Instrument Co. Ltd. The number average molecular weight (Mn) and the molecular weight distribution (MWD) of the block copolymers and the weight molecular weight (Mw) of the homopolymers are listed in Table 1. Analytical CHCl3, ethanol, and acetone were purchased from Tianjin Kermel Chemical Reagent Co. Ltd and used as received.
Preparation of honeycombed films under miscible vapors
The polymer sample was dissolved in CHCl3 to
Results and discussion
The properties of the vapor environment can be tuned by binary mixing water with an organic water-soluble and solvent-soluble vapor with low evaporation enthalpy to assist the polymer film in forming regular pore arrangement. Henceforth, this kind of organic mutual water– and solvent–soluble vapor is defined as additional vapor. In this case, since the additional vapor component is miscible with the solvent in polymer solution, it will enter into the cast solution quickly and cause the polymer
Conclusions
PS–b–PMMA block copolymers with different hydrophobic PS block proportions and total molecular weights, homo–PS, and homo–PMMA have been used to prepare honeycombed morphology through static BF method under additional ethanol/water or acetone/water vapor environment. The influence of the additional vapor component and the parameter of the polymer on the morphology of porous films was investigated. The results suggested that mixed ethanol/water with 20:5 and 20:7 vol ratio vapor was favorable
Acknowledgment
This work is supported by the Liaoning Natural Science Foundation (201602048), the Fund of China Scholarship Council (20163035), and Open Research Fund of State Key Lab-oratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (201715).
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